Receiver input circuits comprising a preamplifier arrangement followed by a mixer arrangement which are connected to one another by adjustable selection means and whose total amplification is controlled and regulated in dependence upon signal input are used, for example, in input circuits for radio and television receivers. With these circuits, it is a question of arriving at a favorable compromise between small and large signal behavior, on the one hand, and low production cost, on the other hand. In the known circuits, this problem is mainly dealt with by appropriate selection of the active semiconductor components and by amplification control. This problem is caused by the limited dynamic characteristics of the passive and active semiconductor components such as bipolar transistors, field-effect transistors and diodes, including tuning diodes.
The Siemens publication "Semiconductor Circuitry Examples" 1972/73, pages 38 to 43, describes circuitry examples for TV tuners dealing with the above-mentioned problem. The block wiring diagram on page 39 of this publication shows, for example, the set-up of a VHF/UHF input section for a TV receiver with an input high-pass filter, followed by a PIN control network, antenna filter for VHF and UHF input sections, uncontrolled preamplifier stages for VHF and UHF and also, for example, band filters tuned with varactor diodes between preamplifier and following mixer stage. Amplification control is effected from a higher input signal level via the PIN diode control network, with the control signal being taken from the IF section of the receiver. This type of amplification control serves to protect the semiconductor components, in this case, bipolar transistors and varactor diodes, from stronger signal drives causing distortions. The basic advantage of such amplification control with PIN diodes lies in the fact that the PIN diodes of the control network themselves cause practically no distortions at all. In the known circuit, however, the noise level increases to the same extent as the control attenuation, causing control actuation to be moved to as high as possible signal levels in order to reach a sufficiently high S/N ratio level with stronger signals at all. In the known circuit, the preamplifier transistor itself is used for amplification control. Here, amplification is lowered by upward control of the collector current of this transistor. The disadvantage of such amplification control is, however, that it entails a non-linearity, dependent on the control condition and partly quite strong, which, in turn, causes signal distortions, inter alia, cross modulation and intermodulation.
Another Siemens publication "Semiconductor Circuitry Examples" 1973/74, page 34, describes a circuit arrangement for the input section of an FM radio receiver with electronically tunable selective circuits. Here, too, amplification control is effected by means of a PIN diode control network between antenna input and the first selective circuit. In this example, the control signal is taken from the IF signal at the output of the circuit, in which case it is recommended to set the control so as to engage only from approximately 1 mV usable signal in order to obtain the maximum S/N ratio level. This type of control serves to prevent overloading of the circuit due to negative outside influences. In the known circuit, the control signal is obtained, relative to the transmission band width, in a narrow band from the signal input to the mixer. This involves the danger of overloading the preliminary and mixer stages with strong signals to which the circuit is not tuned. The mixer stage is particularly endangered here if no or only insufficient amplification step-down control is possible because of the narrow band in which the control signal is gained, and if one or several strong signals reach the mixer stage, amplified or hardly attenuated because of large HF band width and corresponding low selectivity. However, the varactor diodes used in electronic tuning also cause negative influences if they are subjected to strong signals. They themselves then cause cross modulation and intermodulation, for example, and, at certain signal levels and frequencies, can even occasion relaxation oscillations, combined with a strong modulation of the usable signals.
This effect is caused by the dynamic change in the average capacity of the varactor diodes with increasing applied signal voltage. The varactor diodes at the output of the preamplifier stage are affected most, if, as explained above, the preamplifier stage has not or only insufficiently been subjected to step-down control. Even if no relaxation oscillations appear, mistuning of the preselection circuits may occur in the case of a received weak usable signal due to the dynamic capacity changes in the varactor diodes, weakening the usable signal reaching the mixer and thus deteriorating the S/N ratio of the usable signal received. This preselection problem with varactor diodes as tuning reactances is aggravated if it is attempted, under otherwise approximately identical conditions, to obtain a high level of preselection, i.e., trimming the preselection circuits to a high level of resonance quality, a measure which, for instance, appears desirable in view of the large signal characteristics caused by the mixer stage. Such a measure would, on the other hand, intensify possible interference effects, for with identical signal power, a greater signal voltage occurs at the varactor diodes owing to the higher resonance quality.